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ID 48305
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Strontium isotopic age of the Torinosu Limestone in Niyodo Village, Kochi Prefecture, SW Japan.
creator
Tanimizu, Masaharu
Ishikawa, Tsuyoshi
Matsuoka, Jun
Murayama, Masafumi
Kano, Akihiro
subject
strontium isotopic age
Late Jurassic
Early Cretaceous
Torinosu-type limestone
abstract
高知県仁淀村に分布する鳥巣層群谷地層中の石灰岩のSr同位体比は,続成作用の効果を考慮した結果,解像度のよい年代値として利用できることが明らかになった.カソードルミネッセンス法による観察結果と微量元素濃度の分析結果は,石灰岩中の構成物のうち,腕足類化石が最もよく堆積時の初生的な海水Sr同位体比を保持することを示す.また,層孔虫や石灰海綿 Chatetopsis sp.は腕足類よりやや高いSr同位体比を示し,それによる年代のずれは後期ジュラ紀から前期白亜紀においては2.8 m.y.に達するため,これらの化石は詳細な年代の議論に使うことはできない.腕足類から得られたSr同位体比を最新のSr同位体比曲線に合わせると,谷地層下部の石灰岩体の堆積年代は,146.1~148.4 Ma(中期Tithonian)と計算される.Sr同位体比から得られた石灰岩の堆積年代は微化石年代と整合的であり,より解像度が高い.
abstract
Strontium isotopic ratio provides high-resolution ages of the limestone of Yatsuji Formation (Torinosu Group) in Niyodo Village (Kochi Prefecture), on the basis of considering diagenetic effects. Results of cathodoluminescence and trace elements analysis revealed that brachiopod shell is retaining the original marine strontium isotopic value. Stromatoporoids and a calcified sponge Chatetopsis sp. tend to record higher values than the brachiopod shells collected from the same horizon. Their differences are attain to +0.00005, and which corresponds to 2.8 m.y. during late Jurassic to early Cretaceous periods. Therefore, we concluded that stromatoporoids and Chatetopsis sp. are not suitable for the precise age determination. Obtained strontium isotope data of brachiopod shells are converted to depositional age by fitting to recently proposed strontium isotopic curve (LOWESS Look-up Table Version 4). The depositional age of the limestone body of the lower Yatsuji Formation is 146.1-148.4 Ma (middle Tithonian). This age is consistent with microfossil ages, and has higher time resolution.
description
本論は高知大学海洋コア総合研究センター共同利用研究(課題番号 04B011; 代表者,狩野彰宏)の成果である.
journal title
The Journal of the Geological Society of Japan
volume
Volume 111
issue
Issue 10
start page
610
end page
623
date of issued
2005-10
publisher
日本地質学会
issn
0016-7630
1349-9963
ncid
publisher doi
language
jpn
nii type
Journal Article
HU type
Journal Articles
DCMI type
text
format
application/pdf
text version
publisher
rights
© The Geological Society of Japan 2005
relation url
department
Graduate School of Science